Fiber-optic current sensors commonly rely on the Faraday effect in a fused silica fiber. The sensing fiber forms a coil around a conductor that carries the current to be measured. The Faraday effect is observed either as a rotation of the polarization of linearly polarized light or equivalently as a differential phase shift between left and right circularly polarized light waves. In the latter case, the circular light states are commonly produced at the entrance to the sensing fiber by a short section of polarization-maintaining fiber acting as a quarter-wave retarder (QWR). Such a sensor is then designed in a reflective configuration with a mirror at the opposite end of the sensing fiber [1, 2]. Alternatively the sensor may be designed as a Sagnac-type interferometer [2] with QWRs (quarter-wave retarders) at both sensing fiber ends and light waves of the same sense of circular polarization that are counter-propagating in the sensing fiber.
For precise current measurement over a wide temperature range, it is important to maintain the circular polarization states of the light waves in the sensing fiber. Mechanical stress in the fiber, which may be present due to bending of the fiber to a coil, fiber packaging, or hardening of the coating at low temperatures, has a significant influence on the evolution of light polarization in the fiber and may result in an unstable and temperature-dependent signal. Bend-induced stress can limit the possible number of fiber loops, particularly at small loop diameters, to a few loops. As a result the minimum detectable current at a given detection bandwidth (measuring time) is then limited correspondingly.
Thermal annealing of the fiber coils has been used to remove bend-induced stress, but tends to be a sophisticated and time-consuming procedure [2, 3]. In Refs. [4] and [5], the bare fiber resides in a thin capillary of fused silica. The method prevents fiber stress from the coating and fiber packaging. Again the removal of the coating and the insertion of the fiber into a capillary is a time consuming effort.
Laming and Payne showed that the usage of the so-called highly-birefringent spun fiber as a sensing fiber reduces the influence to external mechanical perturbations [6]. Commonly, a spun fiber has a spiral-shaped internal stress field which results in an elliptical birefringence. The parameter of the fiber are commonly chosen so that the eigenmodes of such a fiber are close to left and right circularly polarized light waves. This elliptical birefringence essentially quenches the disturbing effects of linear birefringence, e.g. from bend-induced stress.
The elliptical birefringence may also be produced by an elliptical fiber core that rotates along the fiber [7] or by an adequate fiber micro-structure [8].
Laming and Payne have also demonstrated that the current measurement can delicately depend on the spun fiber parameters, in particular the linear beat length of the corresponding un-spun fiber and the spin pitch, which themselves may vary with temperature [6]. These effects can be reduced by using a broadband light source. Furthermore, Laming and Payne have shown that the angular alignment of the spun fiber with respect to the polarization direction of incoming light has a significant influence on the fiber sensor characteristics,
In Ref [9] the retarder that produces the circular light waves in the (non-spun) sensing fiber was intentionally detuned from perfect 90° retardation. The light waves entering the sensing fiber are then slightly elliptical. If the retarder is detuned by a proper amount, the variation of the sensor scale factor as a result of the temperature dependence of the retarder largely compensates the temperature dependence of the Faraday effect of the non-spun fiber. The sensor signal is then widely independent of temperature. The sensing fiber [9] was an essentially stress-free annealed fiber. In Ref [5] the method was adapted to temperature compensate non-annealed fiber coils having a given amount of bend-induced stress.
Fiber-optic current sensors of the type above can employ a modulator for non-reciprocal modulation of the differential phase of the two interfering light waves in order to operate the interferometer at optimum sensitivity, a technique that has originally been developed for fiber gyroscopes [10].
Alternatively, fiber-optic current sensors can use passive optical elements to generate the phase bias of the light waves. The corresponding conventional sensor configurations employ low birefringent sensing fibers and have been disclosed e.g. in reference [11]. Ref [11] discloses a fiber-optic current sensor where a quarter-wave retarder which is part of a polarization splitter module produces the phase bias; further prior art regarding this sensor configuration can be found in Refs. [12] and [13]. Moreover, Refs. [6] and [14] present prior art on current sensing with highly birefringent spun fibers and purely passive optical detection schemes.